Miscible displacement of petroleum

ABSTRACT

A miscible displacement process for the recovery of petroleum from a petroleum-bearing formation is performed in situ by use of a solvent miscible with the petroleum and having a density greater than water followed by a gaseous driving fluid.

United States Patent [191 Allen et al.

MISCIBLE DISPLACEMENT OF PETROLEUM Inventors: Joseph C. Allen, Bellaire;Jack F.

, Tate, Houston, both of Tex.

Texaco Inc., New York, NY.

May 4, 1973 Assignee:

Filed:

Appl. No.:

US. Cl. 166/274, 166/305 Int. Cl E2lb 43/16 Field of Search 166/267-275,166/305, 306, 308

References Cited UNITED STATES PATENTS 11/1955 Spearow 166/268 [451 Nov.26, 1974 7/1958 Homer 166/268 2,910,123 10/1959 Elkins et al. 166/2713,157,231 11/1964 Darley ..166/268 3,241,614 3/1966 Bertness 166/3043,249,157 5/1966 Brigham et a1. 166/273 Primary Examiner.lames A.Leppink- Attorney, Agent, or Firm-T. H. Whaley; C. G. Ries [57] ABSTRACTA miscible displacement process for the recovery of petroleum from apetroleum-bearing formation is performed in situ by use of a solventmiscible with the petroleum and having a density greater than waterfollowed by a gaseous driving fluid.

12 Claims, 1 Drawing Figure MISCIBLE DISPLACEIVENT OF PETROLEUMBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a process for recovering petroleum by miscible displacement.

2. Description of the Prior Art Various methods for inducing therecovery of petroleum from underground reservoirs are in existence.These methods include injecting water, steam or some aqueous basedmixture to drive the oil from the reservoir. These displacementprocesses are inefficient. The inefficiency of these displacementprocesses is partly due to the retentive forces of capillarity andinterfacial tension. Miscible flooding provides a method for efficientlydisplacing the petroleum from a reservoir.

In miscible flooding, solvent for the petroleum is introduced into thereservoir and driven through the reservoir. Dissolution of the petroleumby the solvent permits no two phase system between the solvent and thepetroleum to exist at the conditions of temperature and pressureexisting in the reservoir. Therefore, the retentive forces ofcapillarity and interfacial tension are nonexistent. These forcesdecrease the displacement efficiency of a recovery process where thedriving fluid or displacing agent and the petroleum exist as two phasesin the reservoir.

In a miscible flood process the solvent has the capability of mixingcompletely with the petroleum in the reservoir. A transition zone isformed at the leading edge of the solvent between the solvent and thepetroleum in which miscibility exists between the solvent and thepetroleum. For economic reasons the solvent is normally injected as aslug followed by another fluid such as a gas or an aqueous fluid todrive the solvent slug and the petroleum through the reservoir.

In displacement processes in general, the ideal sought after ispiston-like displacement. That is, the displacing fluids should ideallypresent a flat front to the petroleum in the reservoir and displace ituniformly through the reservoir. Most miscible solvent slugs arefollowed by an aqueous fluid to drive them through the reservoir.Moreover, most miscible solvents have heretofore been light hydrocarbonswith densities less than water. Problems have arisen with suchprocesses, however.

In a vertical miscible flood, for example, using a light hydrocarbonsolvent slug followed by water, the water will tend to finger throughthe less dense solvent due to viscous fingering and gravity segregation,destroying piston-like displacement and resulting in prematurebreakthrough of the displacing medium water. Further, there are certainpetroleum deposits which are only partially soluble in the prior artsolvents. One type of petroleum which is only partially soluble in priorart solvents are the tar sand oils.

Throughout the world there are various known locations wherein the earthcontains large deposits of tar sands. For example, one of the mostextensive and best known deposits of this type occurs in the Athabascadistrict of Alberta, Canada. In the tar sands in such deposits, the oiltypically has a density approaching or even greater than that of water.The Athabasca tar sands extend for many miles and occur in varyingthicknesses of up to more than 200 feet. Although in some places theAthabasca tar sands are disposed practically on the surface of theearth, generally they are located under an overburden which ranges inthickness from a few feet to as much as 1,000 or more feet in depth. Thetar sands located at these depths constitute one of the worlds largestpresently known petroleum deposits. In these sands, the oil contentranges between about 10 percent and 20 percent by weight, although sandswith lesser or greater amounts of oil content are not unusual.Additionally, the sands generally contain small amounts of water in therange of from about I to 10 percent by weight.

The oil present in and recoverable from Athabasca tar sands is usually arather viscous material ranging in specific gravity from slightly below1.00 to about 1.04 or somewhat greater. At a typical reservoirtemperature, e.g., about 48 F., this oil is immobile, having a viscosityexceeding several thousand centipoises. At higher temperatures, such astemperatures above about 200 F. this oil becomes mobile, withviscosities of less than about 343 centipoises, and the tar sands areincompetent. Since this tarry material does not generally command a veryhigh price, particularly when in its crude state, its separation andrecovery must involve a minimum of expenditure in order to beeconomically attractive for commercial practice.

SUMMARY OF THE INVENTION The invention is a process for the recovery ofpetroleum from a reservoir by miscible displacement. The processinvolves introducing a slug of solvent capable of dissolving formationhydrocarbon and of density greater than water into the upper portion ofa reservoir and forcing the solvent through the reservoir to some pointlower than the point of introduction by means of a driving fluid andproducing the solvent and extracted fluids from the reservoir at thispoint.

BRIEF DESCRIPTION OF THE DRAWING The drawing depicts the vertical driveprocess of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention is animprovement in the displacement of heavy oil, such as tar sand oil. Theimprovement comprises a downward displacement of petroleum with apetroleum solvent having a density greater than a fluid driving thesolvent and preferably a viscosity less than the driving fluid. Thesolvent, being heavier than the driving fluid, will substantiallyeliminate fingering of the driving fluid through the solvent. That is,in a downward drive the lighter driving fluid will ride above theheavier solvent providing a piston-like displacement of the solvent.Also, if the driving fluid is more viscous than the solvent, thelikelihood of fingering of the aqueous fluid through the solvent isfurther reduced. Thus, both gravity stabilization and favorableviscosity contrast is provided between the driving fluid and thepetroleum solvent.

Aqueous fluids such as water are suitable fluids for displacing thesolvent through the reservoir, but any fluid having favorable propertiesmay he used. The displacing fluid must be lighter than the solvent. Thefluid must also be unreactive with the solvent.

Aqueous fluids provide all of the advantages and prevent both viscousfingering and gravity segregation in a vertical drive. However, gravitysegregation alone will provide some protection against fingering of thedriving fluid into the solvent. Consequently, driving fluids lessviscous than the solvents may be used. For example, gases includinglight hydrocarbons and carbon dioxide are suitable driving fluids in theprocess of our invention. Light hydrocarbons include, for example, C andC homologs such as methane, ethane, propane, isobutane and butane.

The types of solvents useful in the process of our invention are thosewhich are more dense than water and chemically inert to the drivingfluid and have solubility characteristics which enable them to dissolveadequate amounts of petroleum. Ideally, the solvent should be completelymiscible with the petroleum so that the interface between the leadingedge of the solvent and the petroleum is removed. Examples of specificsolvents include but are not limited to carbon disulfide and chlorinatedhydrocarbons such as methylene dichloride and carbon tetrachloride. Anysolvent more dense than water may be used.

In certain applications carbon disulfide is the preferred solventbecause of its unique properties or ease of manufacture and recovery. Inthe case of tar sand oil, for example, the bitumen is more soluble incarbon disulfide than in other solvents and certain bitumens may only besoluble to any appreciable extent in carbon disulfide. Also, where therecovered crude is to be catalytically treated in a refinery, forexample, carbon disulfide is preferred. It is a characteristic ofcovalently bonded halogens such as those found in halogenatedhydrocarbons that they tend to poison some refinery catalysts. Carbondisulfide does not and in addition is quite easily removed fromrecovered crude by physical separation processes to be reused again,leaving the crude substantially free of carbon disulfide. Carbondisulfide may also have a great economic advantage over halogenatedhydrocarbons since it may be manufactured by the reaction between coke(carbon) and sulfur. Coke and sulfur are often found in excess nearprolific tar sand deposits such as the Athabasca tar sands of Canada.The use of these materials would be an aid to conservation of theenvironment.

It is also within the scope of our invention to use as a solvent a blendof carbon disulfide with another component, mutually soluble in carbondisulfide such as a chlorinated hydrocarbon. These materials should alsobe easily removed from dissolved tar sand oilby physical separationtechniques such as vacuum distillation.

The process of our invention may be carried out by a variety oftechniques. In one technique, for example, at least two wells areneeded, one for injection and another for production. The solvent mayproceed through the formation horizontally from the injection well tothe production well, but the benefits of this invention become greateras the angle the solvent proceeds through the formation approaches 90from the horizontal. Ideally, the interface between the solvent andfluid driving it should be horizontal. This configuration allows gravitystabilization to have its maximum effect on the system.

The process of this invention is operable in a variety of petroleumreservoirs containing petroleum of widely differing gravities. Onepreferred embodiment, however, is to recover tar sand oil using asolvent comprising a major amount of carbon disulfide. As pointed outpreviously, it is a characteristic of the bitumen constituents of tarsand oil or petroleum that they are soluble in carbon disulfide and lesssoluble or insoluble in most other solvents.

A very important advantage of using carbon disulfide is the lack of anemulsification of the separate water and carbon disulfide phaseswhenwater is used as the driving fluid. The phases separate into distinctlayers easily separable from each other. This feature is advantageousfor many reasons. For example. emulsification within the formation couldlead to a reduction in permeability due to what is commonly known as"emulsion blockage. The lack of emulsification when carbon disulfide isused prevents this problem from occurring. Also, emulsification coulddestroy piston-like displacement. Another advantage of the lack ofemulsion forming tendency between carbon disulfide and water occurs whenthe solvent, bitumen and water are produced and separation of the carbondisulfide is desired. Emulsion formation would distinctly hamper theseoperations.

The size of solvent slug to be used will depend on the solvent chosenand the degree of recovery desired. The degree of recovery desired is amatter of economics and may be determined by those skilled in the artwithout engaging in inventive effort. As an aid in determining the sizeof slug needed the following procedure may be used but is not intendedto limit the scope of our invention or tie it to any routine calculationprocedure. The size of a slug of carbon disulfide, for example, may becalculated by a formula such as:

Solubility of bitumen amount of bitumen in carbon disulfide (CS X peracre-foot of formation acre-feet degree of depletion desired X information X (decimal) amount of carbon disulfide required Routinelaboratory experimentation may be used to determine the solubility of agiven bitumen in carbon disulfide and core analysis will yieldinformation on the amount of bitumen per acre foot of formation. Thus,the size of solvent slug for any field may be determined.

The temperature of the solvent slug should be low enough to avoid havingthe carbon disulfide react with water when water is the driving fluid.Normally the slug will be introduced at ambient temperature and willtake on the reservoir temperature in a short period of time. In onelocation of the Canadian tar sands, for example, the reservoirtemperature is about 45 F. In no case when carbon disulfide is used withwater should the temperature of the solvent be above that at whichcarbon disulfide reacts with water, about 400 F.

The temperature of the displacing or driving water should not be so lowthat, in combination with dissolved salt content, its density exceedsthat of the solvent.

A fairly thick reservoir is preferred in the process of this inventionto allow as near to a vertical miscible flood as possible. The placementof the injection and production wells is related. They should besituated so that the injection of the solvent and the drive fluid takesplace at a point in the reservoir above the point where the productionis taken from the reservoir. The lateral as well as vertical spacing ofthe production and injection points should be such that a blanket ofsolvent followed by drive fluid will cover the largest area of thereservoir consistent with economics. The above factors should beunderstood as given to explain how to maximize the effectiveness of thisinvention. However, the invention should not be construed as limited to.any particular well configuration or reservoir type.

MINING TECHNIQUES In one embodiment of the invention a dump flood may beperformed. This involves drilling a large diameter hole into the crestalportion of a tar sand formation, for example. A solvent heavier thanwater, such as carbon disulfide and/or carbon tetrachloride or mixturesthereof, is dumped into the cavity. The solvent will gravitate into theformation and be imbibed by the formation displacing the oil towardproducing wells completed lower in the formation. An aqueous fluid suchas water is introduced into the cavity to maintain a layer over thesolvent. This prevents evaporation of the solvent. Dump flooding isparticularly useful where the formation is near to the surface of theearth where high injection pressures could result in breakthrough ofinjected fluids through the overburden to the surface. Near to thesurface could include, for example, depths of 200 to 300 feet or more.

A similar mining technique could also be used where the tar sandsoutcrop at the surface. A dam of earth, for example, could beconstructed surrounding the outcrop providing a recepticle for theinjection of solvent and water. Production wells drilled down dip fromthe outcrop would withdraw the combined solvent-tar extract.

As a rule of thumb so-called mining techniques are normally consideredto be feasible where the ratio of the distance from the surface of theearth to the thickness of the tar sand reservoir or pay is one or less.

IN SITU RECOVERY TECHNIQUES For deeper formations other embodiments ofour invention involve having at least one production well and oneinjection well. The preferred configuration is that the point ofinjection be far enough above the point of production to allow asomewhat vertical traverse for fluids entering the reservoir through theinjection well and being produced through the production well. A slug ofsolvent heavier than water, carbon disulfide, for

'example, is introduced through the injection well followed by a drivingfluid to push the solvent through the reservoir to the production well.

A variation of this includes using one well having upper and lowerperforation and which is internally equipped so as to avoid fluidcommunication in the well between the upper and lower perforations. Theheavy solvent is injected through the upper perforations and producedthrough the lower perforations along with petroleum driven ahead of thesolvent. If desired, the solvent may be followed by a driving fluid.

The process of our invention may be illustrated by reference to theaccompanying figure which depicts one embodiment of our invention. Otherembodiments will, of course, occur to those having had the benefit ofthe teachings contained herein.

A reservoir containing a very viscous petroleum is penetrated by aninjection well 11 and production wells 12 and 13, the injection wellhaving communication with the reservoir through perforations 14 abovethe perforations 15 in the injection wells. The figure represents apoint in time well into the recovery program where a slug of carbondisulfide 16 has been injected into the reservoir through the injectionwell followed by carbon dioxide l7 which is presently being injectedinto the injection well. Thepetroleum 18 is moving towards theproduction well perforations where it is produced. The interface shownbetween the carbon disulfide slug l6 and the petroleum 18 or the carbondioxide driving fluid 17 is, of course, not as distinct as shown in thisillustration.

EXPERIMENTAL The superior performance of a carbon disulfide slug inremoving tar sand oil has been demonstrated in laboratory experiments. Aweighed quantity of tar sand was placed in a glass tube 1.5 inches indiameter above a glass wool filter to retain the-sand. Sufficient carbondisulfide was introduced into the tube to completely saturate and coverthe tar sand. Water was then placed above the carbon disulfide-tar sandlayer. The water formed a distinct layer above the carbon disulfide-tarsand system. The assembly was closed and shut-in over night. The nextday a stopcock at the bottom of the assembly was opened and all fluidallowed to drain out into a graduate cylinder. A two phase system formedin the graduate cylinder: a lower phase consisting of carbon disulfideand dissolved tar sand oil, and an upper clear water phase. The waterpassed easily through the tar sand and was drawn off easily since thephase boundary was distinct. No emulsion formation was noted at theinterface between the water and the carbon disulfide-tar sand oilmixture. Virtually all of the tar and injected carbon disulfide wererecovered from the original tube.

The carbon disulfide extracted tar oil mixture was vacuum distilled. Nofrothing or foaming occurred during the operation even though thesolution actually boiled. All of the carbon disulfide was removed andonly pure, heavy, viscous tar oil remained.

We claim:

1. A process for recovering petroleum from a reservoir comprising:

a. introducing a slug of solvent for the petroleum into the reservoirwhich solvent has a density greater than water,

b. introducing, following the solvent, a gaseous driving fluid into thereservoir, to force the solvent through the reservoir wherein theinterface between the solvent and the driving fluid approaches asubstantially horizontal position, and

c. producing the petroleum and solvent at a point below the point ofintroduction of the solvent.

2. A process as in claim 1 wherein the driving fluid is a gaseousmaterial selected from the group consisting of carbon dioxide oraliphatic hydrocarbons having from one to four carbon atoms.

3. A process as in claim 2 wherein the reservoir is a tar sandreservoir.

4. A process as in claim 3 wherein the solvent comprises carbondisulfide.

5. A process as in claim 3 wherein the solvent comprises a chlorinatedhydrocarbon.

6. A process as in claim 3 wherein the solvent comprises a mixture ofcarbon disulfide and a chlorinated hydrocarbon.

7. A process for recovering petroleum from a subterranean reservoirwherein there is at least one injection well penetrating and incommunication with the reservoir and at least one production wellpenetrating and in communication with the reservoir at a point below thepoint of communication of the injection well which comprises:

a. introducing a slug of solvent for the petroleum into the reservoirvia the injection well which solvent has a density greater than water,

b. introducing, following the solvent, a gaseous driving fluid into thereservoir, to force the solvent through the reservoir wherein theinterface be- 10 tween the solvent and the driving fluid approaches asubstantially horizontal position, and

c. removing the petroleum and solvent through the production well.

8. A process as in claim 7 wherein the reservoir is a tar sandreservoir.

9. A process as in claim 8 wherein the solvent comintroduction of thecarbon disulfide.

1. A PROCESS FR RECOVERING PETROLEUM FROM A RESERVOIR COMPRISING: A.INTRODUCING A SLUG OF SOLVENT FOR THE PETROLEUM INTO THE RESERVOIR WHICHSOLVENT HAS A DENSITY GREATER THAN WATER, B. INTRODUCING, FOLLOWING THESOLVENT, A GASEOUS DRIVING FLUID INTO THE RESERVOIR, TO FORCE THESOLVENT THROUGH THE RESERVOIR WHEREIN THE INTERFACE BETWEEN THE SOLVENTAND THE DRIVING FLUID APPROACHES A SUBSTANTIALLY HORIZONTAL POSITION,AND C. PRODUCING THE PETROLEUM AND SOLVENT AT A POINT BELOW THE POINT OFINTRODUCTION OF THE SOLVENT.
 2. A process as in claim 1 wherein thedriving fluid is a gaseous material selected from the group consistingof carbon dioxide or aliphatic hydrocarbons having from one to fourcarbon atoms.
 3. A process as in claim 2 wherein the reservoir is a tarsand reservoir.
 4. A process as in claim 3 wherein the solvent comprisescarbon disulfide.
 5. A process as in claim 3 wherein the solventcomprises a chlorinated hydrocarbon.
 6. A process as in claim 3 whereinthe solvent comprises a mixture of carbon disulfide and a chlorinatedhydrocarbon.
 7. A process for recovering petroleum from a subterraneanreservoir wherein there is at least one injection well penetrating andin communication with the reservoir and at least one production wellpenetrating and in communication with the reservoir at a point below thepoint of communication of the injection well which comprises: a.introducing a slug of solvent for the petroleum into the reservoir viathe injection well which solvent has a density greater than water, b.introducing, following the solvent, a gaseous driving fluid into thereservoir, to force the solvent through the reservoir wherein theinterface between the solvent and the driving fluid approaches asubstantially horizontal position, and c. removing the petroleum andsolvent through the production well.
 8. A process as in claim 7 whereinthe reservoir is a tar sand reservoir.
 9. A process as in claim 8wherein the solvent comprises carbon disulfide.
 10. A process as inclaim 8 wherein the solvent comprises a chlorinated hydrocarbon.
 11. Aprocess as in claim 8 wherein the solvent comprises a mixture of carbondisulfide and a chlorinated hydrocarbon.
 12. A process for recoveringpetroleum from a reservoir comprising: a. introducing a slug of carbondisulfide into the reservoir; b. introducing, following the carbondisulfide, a gaseous driving fluid into the reservoir, to force thecarbon disulfide through the reservoir, and c. producing petroleum at apoint below the point of introduction of the carbon disulfide.